Of all the men who attacked the
flying problem in the 19th century, Otto Lilienthal was easily the most important. His
greatness appeared in every phase of the problem. No one equaled him in power to draw new
recruits to the cause; no one equaled him in fullness and dearness of understanding of the
principles of flight; no one did so much convince the world of the advantages of curved
wing surfaces; and no one did so much to transfer the problem of human flight to the open
air where it belonged. As a missionary he was wonderful. He presented the cause of human
flight to his readers so earnestly, so attractively, and so convincingly that it was
difficult for anyone to resist the temptation to make an attempt at it himself, even
though his sober judgment and the misfortunes of all predecessors warned him to avoid
touching it. If Lilienthal had done nothing more than this he still would have been one of
the greatest contributors to the final success. But he was much more than a mere
missionary. As a scientific investigator none of his contemporaries was his equal. He set
forth the advantages of arched wings in such convincing manner as to make him the real
originator of this feature. Others had noted that bird wings were arched, and had
speculated on the possibility that an arched wing was superior to an absolutely true
plane, but Lilienthal demonstrated the reason why it was better, and changed mere
speculation into accepted knowledge. He also devoted an enormous amount of time and
patience to experiments with test surfaces for the purpose of determining the best shapes
for wings and for the amount of pressures to be obtained at the various angles of
incidence. For nearly twenty years his tables and charts were the best to be found in
print. His work in this line alone would have been sufficient to place Lilienthal in the
front rank, yet there still remains to be mentioned his greatest contribution to the
cause. Lilienthal was the real founder of out-of-door experimenting. It is true that
attempts at gliding had been made hundreds of years before him, and that in the nineteenth
century, Cayley, Spencer, Wenham, Mouillard, and many others were reported to have made
feeble attempts to glide, but their failures were so complete that nothing of value
resulted.

Lilienthal pursued the undertaking so persistently and
intelligently that although his own death and that of Pilcher for a time caused a
cessation of this mode of experiment, nevertheless his efforts constituted the greatest
contribution to final success that had been made by any of the nineteenth century group of
workers.

When the general excellence of the work of Lilienthal is
considered, the question arises as to whether or not he would have solved the problem of
human flight if his untimely death in 1896 had not interrupted his efforts. Many people
believe that success was almost within his grasp. Others think that he had limitations
which rendered such an outcome at least doubtful. One of the greatest difficulties of the
problem has been little understood by the world at large. This was the fact that those who
aspired to solve the problem were constantly pursued by expense, danger, and time. In
order to succeed it was not only necessary to make progress, but it was necessary to make
progress at a sufficient rate to reach the goal before money gave out, or before accident
intervened, or before the portion of life allowable for such work was past. The problem
was so vast and many sided that no one could hope to win unless he possessed unusual
ability to grasp the essential points, and ignore the nonessentials. It was necessary to
have a genius for solving almost innumerable difficult problems with a minimum expenditure
of time, a minimum expenditure of money, and a minimum risk of accident. A study of the
failures of the nineteenth century shows clearly that none of the important workers stood
still, but that the rate of progress was so slow that each one was overcome and removed
from the race by one of the causes just mentioned before the goal was reached. If they had
possessed the faculty of doing things more quickly, more simply, and less expensively,
they might not have been overtaken by old age, lack of funds, or accident. Some were
traveling at a rate which would have required fifty years or more to reach success. Others
were spending money at a rate which would have necessitated an expenditure of millions of
dollars in order to complete the task. When the detailed story is written of the means by
which success in human flight was finally attained, it will be seen that this success was
not won by spending more time than others had spent, nor by spending more money than
others had spent, nor by taking greater risks than others had taken.

Those who failed for lack of time had already used more
time than was necessary; those who failed for lack of money had already spent more money
than was necessary; and those who were cut off by accident had previously enjoyed many
lucky escapes as reasonably could be expected.

Lilienthal progressed, but not very rapidly. His tables of
pressures and resistances of arched aeroplane surfaces were the results of years of
experiment and were the best in existence, yet they were not sufficiently accurate to
enable anyone to construct a machine with full assurance that it would give exatly the
expected results. Under such conditions progress could not but be slow. His methods of
controlling balance both laterally and longitudinally were exceedingly crude and quite
insufficient. Although he experimented for six successive years 1891-1896 with gliding
machines, he was using at the end the same inadequate method of control with which he
started. His rate of progress during these years makes it doubtful whether he would have
achieved full success in the near future if his life had been spared, but whatever his
limitations may have been, he was without question the greatest of the precursors, and the
world owes to him a great debt.